1. Field of the Invention
This invention relates to a machine to generate smoke from wood or similar organic matter. The smoke produced is intended for supply to food smoking kilns. The smoke generator could also be used to generate smoke from a variety of combustible materials for other conceivable practical purposes or for research purposes.
This invention provides the capacity to produce smoke with control over the major parameters of smoke generation. The smoke would be produced from organic matter such as wood particles, and would be used for supply to food smoking kilns or for other purposes, either practical or scientific. With this machine, the effects that the parameters of smoke generation have on the properties of the smoke with respect to its use for food smoking could be studied in a scientific manner. Smoke generation parameters could be selected and controlled so that the smoke generator would supply smoke that has been produced under conditions optimizing the desirable components and qualities and minimizing the undesirable constituents in the smoke for food curing. The smoke generation parameters and smoke qualities could be held consistent for any desired period of operation. The smoke generator also has the capacity to selectively vary the different generation parameters independently.
Smoke can be generated in a concentrated form with little or no dilution by air or other added gases with this design of smoke generator, while in typical smoke generation practice, the combustion reactions that begin during smoke formation must be quenched by cooling the smoke gases by dilution with ambient air or some other cool gas, resulting in the production of diluted smoke.
The generator has the capacity to rapidly vary the rate of smoke production. Since the smoke generation parameters can be automatically monitored and controlled by computer, the generator could quickly control the smoke supply in response to the requirements of the downstream food smoking process. With this smoke generator, both the rate of smoke generation and the parameters of smoke generation can be monitored and controlled by a process controller and computer.
This smoke generator design has research as well as commercial applications.
2. Description of the Prior Art
Traditionally and still in common practice, smoke for food curing has been generated from smouldering sawdust fires. Other techniques for smoke generation have been devised and employed either for research or commercial applications. A description of the smouldering process will be given here followed by a discussion of some of the previous alternate techniques devised for smoke generation. To date, it appears that none of these previous methods for smoke production can provide the level of control over smoke generation parameters such as can be achieved by the smoke generator design described herein.
Combustion of a fuel source refers to the process of oxidation with the evolution of heat and light, while smouldering refers to incomplete combustion that produces smoke but no visible flame. Typically, the combustion of wood takes place in two stages. Initially heat input to the wood is required to thermally degrade the wood components. This releases gaseous hydrocarbons that combust exothermally at some distance from the wood surface producing a visible flame. As the solid wood is pyrolysed, char remains. The char may support a "glowing combustion" within the solid wood. Too little or too much air may limit these combustion processes. Excess air supply to the zone where combustion of the gases occurs may dissipate so much heat that the initial endothermic pyrolytic reactions are inhibited.
Smoke is produced in large quantities under conditions of inefficient combustion that exist during smouldering. The initial stage of smoke formation begins with the thermal breakdown of the chemical bonds in the macromolecules and polymers of wood. The gases released by these pyrolytic reactions in the wood are exposed to atmospheric oxygen in an oxidative zone surrounding the wood surfaces. The pyrolytic reactions that release gaseous hydrocarbons from wood are mainly endothermic and do not provide sufficient heat energy to maintain the pyrolysis. In the absence of an external thermal energy source, the heat from exothermic oxidation reactions is required to maintain the thermal degradation and volatilization of the wood. The oxidation of the "char" residues of smouldering solids also contributes to the energy input required to drive the endothermic pyrolytic reactions. Smouldering usually occurs in fuels with large surface to volume ratios that promote oxidation reactions at the solid surfaces and favour quenching of the gas-phase oxidative reactions by heat dissipation and gas diffusion. Sawdust is an example of a fuel with a large surface to volume ratio that favours the smouldering process.
Temperatures of 860.degree. C. to 940.degree. C. were typical in the glowing zone of smouldering sawdust burning with a natural air draft. Other researchers believed there was evidence to support the concept that during the initial pyrolytic stage of wood degradation all the known compounds found in wood smoke are formed, and the oxidative reactions probably produced only a quantitative change in the relative amounts of the various air-borne smoke chemicals. From a review of wood smoke generation technology, these researchers concluded that, ". . . we are not entitled to say that this wild process, which the self-propagatory combustion of wood indeed is, has been tamed".
Many smoke generators simply employ sawdust beds arranged to sustain a smouldering process by natural air draft. A further common development upon this natural process has been to employ a heat source such as an electric resistance heat element to provide additional heat energy to sustain pyrolytic reactions required for the thermal degradation of wood to form gaseous hydrocarbons.
U.S. Pat. No. 4,270,464 (Kerres, 1981), describes a smoke generator that controls the flow rate of sawdust into a smoke generating chamber that provides a heating element to ignite the sawdust. Excess oxygen is supplied to the smoke generating chamber by a mechanical fan to support smouldering combustion; smoke is produced from glowing sawdust maintained at a temperature just below the flame point in the presence of excess oxygen.
In the Kerres invention, smoke is generated in a single location within the smoke generating chamber. The processes of pyrolysis of wood particles and partial combustion of pyrolytic products occur together at the same location in more or less the same fashion as occurs with a natural smouldering sawdust fire.
Kerres' invention does not physically separate the processes of pyrolysis and combustion reactions required to provide precise operator or computer control over the pyrolysis temperatures in the pyrolysis stage and the levels and temperatures of air supply to the partial combustion stage. Kerres' invention provides an apparatus for sustained consistent production of smoke from a typical smouldering sawdust fire.
Reviews by other researchers cited various other techniques developed for smoke generation that included: (1) friction smoke generation; (2) steam smoke generation; (3) fluidized-bed; (4) two stage generation; (5) isothermal; and (6) carbonization smoke generation.
The friction smoke technique for smoke generation is accomplished by pressing a stick of wood against a rapidly rotating steel wheel or cylinder. The friction between the wheel and the wood generates the heat for pyrolysis. Oxygen for secondary reactions can be supplied through holes in the surface of the metal friction wheel. Pyrolysis temperatures are in the range of 450.degree. C. to 560.degree. C. (lower than most natural smouldering temperatures) and rapid cooling of the smoke limits the secondary oxidative reactions. Meat products treated with friction smoke were found to have different sensory properties from meats smoked by a smouldering source.
Smoke generation was also developed with a system in which pyrolysis was induced by delivering superheated steam at 300.degree. C. to 400.degree. C. containing a small amount of oxygen through a bed of wood chips. Oxygen was required to induce the secondary reactions in smoke chemicals, but the level of oxygen had to be limited since the pyrolysis temperatures increased above the superheated steam temperatures when the oxidative reactions increased.
One researcher has devised a fluidized-bed technique for smoke generation. Air heated between 300.degree. C. to 400.degree. C. was used to suspend a bed of wood particles. The temperature of pyrolysis was difficult to control but high efficiency of wood conversion to smoke components was achieved.
Other researchers described a two stage smoke generator that was further refined by another researcher. In the first stage, pyrolysis of wood was induced by a stream of hot inert gas. The second stage involved pyrolysis at a controlled rate and temperature by addition of a regulated stream of heated air. This system was used to examine the effects of pyrolysis and oxidation temperatures on the quality of smoke used for food curing. It involved the incorporation of large volumes of inert gases and air into the final smoke.
Another researcher developed an isothermal smoke generator that fed sawdust by an auger through a pipe heated by electric elements. Oxygen was limited but not accurately controlled. In order to control the temperature, the pyrolysis zone had to be kept below 500.degree. C. to prevent run-away heating due to exothermal reactions at higher temperatures.
The carbonization process for smoke generation involved compressing sawdust in a tubular casing with a tapered screw. Most of the air in the sawdust is eliminated and a variable temperature heating element at the end of the screw casing causes the production of smoke during carbonization.
A German patent application described a process for generating food curing smoke by direct microwave pyrolysis. It was claimed that the pyrolysis temperature was always maintained below 400.degree. C. without thermostatic control and the resulting smoke had low concentrations of undesirable components.
All of these different types of smoke generators produced smoke with variations in moisture content, oxygen content and ratios of chemical components. However, researchers noted that in all cases, researchers compared their results to foods cured with smoke produced by the traditional smouldering process.
Various researchers have reported that changing the parameters of smoke generation (such things as the pyrolysis temperature, the level of partial combustion, the initial moisture level of the wood, and the wood species) changed the relative balance of chemicals in the smoke. Some work has been done to identify smoke generation parameters that would optimize the levels of desirable components in wood smoke and minimize the less desirable chemicals. A review of these attempts indicates there has been to date a lack of defined control over the relevant parameters for smoke generation. This has prevented accurate specification of optimal smoke generation parameters. This innovative smoke generator design will enable research to define optimal smoke generation parameters for food curing purposes. Subsequently, these generation parameters will be readily reproduced in commercial food smoking processes by the application of this smoke generator design.